The present invention is directed to a method for manufacturing a buried region of increased refractive index in a glass member by an ion exchange. The method comprises the steps of providing a mask of titanium on a surface of a glass member or substrate, raising the refractive index of exposed portions of the glass surface by penetrating the exposed surface with an iontype raising the refractive index of the glass, then removing the titanium mask and then burying the raised refractive index portions in the glass member or substrate by conducting a second ion exchange with another type of ion, which will not raise the refractive index but causes an inward migration of the ions that raise the refractive index.
It is known to raise the refractive index of glass in a method wherein Cs.sup.+ ions from a CsNO.sub.3 /KNO.sub.3 melt in a first fieldpromoted ion exchange are used. Thus, the Cs.sup.+ ions penetrate into the glass only where the region of raised refractive index, for example, a strip waveguide or a structure composed of such waveguides, is to occur. To control this, a mask of titanium is employed, and the titanium will block the penetration of the Cs.sup.+ ions into the glass on those portions of the surface covered by the titanium mask. The regions of raised refractive index will be produced in those regions not covered by the layer of titanium material or, in other words, the spaces between the layers or portions of titanium material. A similar method is known from an article in Appl. Optics Vol. 23, No. 11 (1984), pp. 1745.
An additional field-assisted ion exchange for burying the region is then undertaken. For example, the additional ion exchange uses a salt melt which contains an alkali ion of the glass, generally Na.sup.+ or/and K.sup.+ ions. In this exchange, the region of raised refractive index produced by the first ion exchange will migrate into the interior of the glass member and is, thus, buried.
Problems occur in the removal of the roughly 200-500 nm thick mask of titanium from the surface of the glass member or substrate. One problem is to find an etchant, which will etch the titanium away without damaging the glass surface or damaging the region of the raised refractive index, for example, the strip waveguide or the strip waveguide structure.
Due to chemical reactions in the hot salt melt, the mask of the titanium is significantly more resistant to a hydrofluoric acid than in the case of a fresh layer of titanium. A mechanical erosion, for example by grinding of the titanium, is also a problem, because the glass member may have warped due to the treatment with the hot salt melt, and an adequately planar surface generally no longer exists. Therefore, problems occur with being able to grind or polish the mask away without damaging or destroying the region of the raised refractive index.